When one plate sits perpendicular to another, as shown above, you have a T-joint. The two sides are welded where they meet, either on one side or both sides, depending on the product specs or construction plans. You'll find that T-joints are extremely common in structural welding. A fillet weld is usually used for a T-joint, but occasionally the specs call for some beveling. In that case, you'll fill the joint with a groove weld. Unlike the way the diagram above depicts it, the weld for a T-joint should penetrate into both work plates, fusing them together. To assure good penetration, the vertical plate can be beveled in different ways prior to welding, as shown below. The most common T-joint is the first example, known as a multi-pass fillet weld. Although the next section of this tutorial covers welding symbols in details, here's a head-start: When a weld type (fillet, groove, etc.) appears on the bottom side of the horizontal line, it means the weld is performed closest to the arrow. Where you see the weld shape depicted on top of the line, it means the weld is performed on the other side of the plate, away from the arrow. When the shape is included above and below the horizontal line in the symbol, both sides of the joint must be welded. Two metal plates that meet along one side comprise a butt joint. The weld may penetrate all the way through, as shown in the drawing above, or it may only have partial penetration. Either way, there needs to be a gap between the plates before you start the weld. The thicker the plates, the greater the gap. For plate thicknesses of more than a quarter inch, a bevel or V-groove type weld is usually required to get the job done. Here are some of the more common specs for butt joints: Lap joints are not welded where the two faces meet, but on one or both edges that bookend the faces. Generally, a fillet weld will do the job for these joints, but it's important to have good fit-up so the two faces sit flat against each other. An edge joint looks a little like a corner joint. The difference is that the two sides don't joint at a perpendicular angle, but instead share the same plane (or something akin to it). Corner joints bring together two edges of the work plates that meet at a perpendicular angle, or along two different planes. (When the plates lie on the same plane, you have either a butt joint or edge joint.) As illustrated in the drawing on the left, a corner joint may be open (top) or closed (bottom). The two sides are typically joined by a fillet weld.